如何处理并发事务？

Question

I am using Glassfish 3.1 + Hibernate 3.6 + Postgres 9 and i am not fully understanding how i should handle "long" running concurrent transactions.

My transactions can take up to one second in some cases. What happens is that i generate objects out of a template table when i receive an event from a client. It can happen that two different events produce an overlapping set of objects.

I am thinking about this scenario:

T1 begin
T1 read objects which may already exist

T2 begin
T2 select templates which have to be processed
T1 inserts (generate objects)
T1 commit

T1 select templates which have to be processed
T1 inserts (generate objects)
T1 commit

How will this be propagated to me? Any lock exception? Or will i get a constraint exception because of an unique index? How can i handle this situation? Restart an event processing 3 times before stop processing it?

greetings, m

Answer 1

The way that PostgreSQL handles this is called multi-version concurrency control. Basically concurrent transactions see a snapshot of the data when they begin the statement (by default) or more seldom when they begin the transaction (depending on transaction level).

In this approach readers never block writers, and writers only block readers when the reader requests it. For just reading and inserting you are unlikely to get lock exceptions/